The present invention relates to a process for the isolation of L-amino acids from the cleavage solution produced as a result of the acylase-catalyzed cleavage reaction of N-acetyl-D,L-amino acids using a strongly acidic ion exchanger in the H+ form.
The essential principle of the acylase-catalyzed cleavage reaction of N-acetyl-D,L-amino acids into L-amino acid, acetic acid and N-acetyl-D-amino acid are known in the art. The cleavage reaction is traditionally undertaken in a solution of the N-acetyl-D,L-amino acid which is between a 0.4 and a 1.0 molar solution and at a pH of between 6 and 8. The N-acetyl-D,L-amino acid is customarily used in the form of the sodium or potassium salt.
It is also known in the art to isolate the L-amino acid from the cleavage solution by evaporation and final cooling down to crystalline form. The drawback of this direct treatment of the crude cleavage solution resides in the fact that sodium or potassium acetate salts are also precipitated, together with the L-amino acid, and this contaminates the L-amino acid.
Following the separation of the crystallized L-amino acid, the mother liquor remaining is processed by commercially known effluent technology processes for recovery of N-acetyl-D,L-amino acid. With the direct processing of the crude cleavage solution, the mother liquor also contains among other things considerable quantities of the L-amino acid. If this mother liquor is to be processed into N-acetyl-D,L-amino acid, as a first step, the L-amino acid which is contained therein must be newly acetylated, because otherwise peptides are formed during the re-racemization of the N-acetyl-D-amino acid. This means that the L-amino acid which has been released by acylase must be subjected again, after acetylization and re-racemization, to another acylase-catalyzed cleavage. Thus, the necessary quantity of acylase is increased and an additional product loss occurs with the re-racemization.
Finally, it is also known to isolate the L-amino acid which is contained in the crude cleavage solution with use of a strongly acidic ion exchanger in the H+ form. In that process, a sufficient quantity of cleavage solution is fed to the ion exchanger so that the ion exchange material is loaded up to 80 to 85% of its capacity with alkali metal ions and L-amino acid molecules. Following washing out with water, the L-amino acid is eluted from the ion exchanger with an aqueous ammonia solution. As a result of the evaporation and subsequent cooling down, the L-amino acid is obtained from the eluate in crystalline form. If an approximately 3N ammonia solution is used for the elution, alkali metal ions are also partially obtained in the eluate and these ions lead to an increased ash content in the crystallized L-amino acid product. This drawback can now actually be overcome to a great extent by use of a diluted ammonia solution, for instance an 0.4 or 0.5N solution. Then, however, a very diluted eluate is obtained, which requires an extended evaporation time, which then again leads to increased by-product formation and/or racemization of the L-amino acid. Another drawback of this known process resides in that the ion exchanger can only be used up to approximately 45 to 50% of its capacity for the bonding of alkali metal ions.